Light detectors



Nov. 7, 1961 c. F. HENDEE LIGHT DETECTORS Filed Dec. 23, 1953 3Sheets-Sheet 1 Bxl/@WWW AGENT Nov. 7, 1961 c. F. HENDEE LIGHT DETECTORSFiled Dec. 25, 1953 3 Sheets-Sheet 2 11W/nvm;

Iimmllv' @ARLES Nov. 7, 1961 C. F. HENDEE LIGHT DETECTORS 3 Sheets-Sheet3 States are Filed Dec. 23, 1953, Ser. No. 399,870' 7 Claims. (Cl.Z50-207) This invention relates -to a method of and apparatus fordetecting light radiation and more particularly to irnprovedphotomultiplier -tube circuits for determining the intensity of anincident flash of light which has an intensity far below the light leveldetectable by normal direct voltage operation of photomultiplier tubes.

The conventional photomultiplier tube is a high vacuum device in whichthe photo-current produced at a lightsensitive cathode is multipliedmany times by secondary emission occurring at successive intermediateelectrodes or dynodes prior to reaching the collector or anode of thetube. --For normal operation of a photomultiplier tube, a ratedoperative value of constant direct voltage is irnpressed on thesuccessive tube electrodes to direct electron ow in a direction from thelight-sensitive cathode to the anode via each of the dynodes. Theresultant output current is alinear function of the incidentillumination on the cathode.

However, photomultiplierV tubes are notoriously noisy and the noise isparticularly undesirable when the tubes are used to detect low lightlevels. A recognized'source of such noise is the thermal emission ofelectrons from the photocathode and dynodes of the tube. Thermalemission of electrons is a sporadic emission of electrons from the tubeelectrodes which occurs at room temperature but which may increase ordecrease depending upon the increase or decrease, respectively, of theelectrode temperature. Ordinarily when the tubes are used at very lowlight levels, it is customary to cool them, for example, with Dry Ice,to low temperatures in order to reduce the noise caused by the thermalelectrons and thereby to enhance the precision of measurement by a morefavorable signal-to-noise ratio. But such cooling is relativelyexpensive and cumbersome and, in many cases, inconvenient.

In many circuits which utilize the output from photomultiplier tubes,operating signals stronger than those yielded at the output of aconventional direct-voltage operated photomultiplier tube are required.The desired signal strength may sometimes be obtained by feeding theoutput of the photomultiplier tube to an amplifying circuit or, moreconveniently, by merely increasing the value of the direct voltage atthe tube electrodes. However, when the value of the direct voltage atthe electrodes is elevated beyond the rated value, gas-generated noiseis likely to occur in the tube. This noise is introduced as a result ofexcessive bombardment of the dynodes and anode by high velocityelectrons which release positive particles, or ions, from the bombardedelectrodes. These ions ow toward the photocathode and strikeintermediate dynodes which release electrons therefrom to produce anunwanted` signal in the output of the tube. The production ofgas-generated noise is usually the limiting factor with respect to thedegree of gain which can be realized from conventional photornultipliertube circuits employing a direct voltage on the tube electrodes.

Circuits employing a photomultiplier tube have been used to analyze theintensity of a ash of light as a function of time. This may beaccomplished by observing the output of a photo-tube or photomultiplieron the screen of an oscilloscope, if sulicient gain is provided betweenthe photo-emitting surface and the oscilloscope and, if in addition, thelight intensity at the photo-emitting surface is greatenough to providea relatively large number of photo-electrons during any interval of theflash. If either of these two conditions is not met, this method is notfeasible.

Y When the intensity of a light ash is low, a photomultiplier tubeoperating at its rated direct voltage may not be able to determine theintensity or even the presence of the light due to the masking elect ofthe thermal noise in the tube.

The principal object of the invention is to provide a method andapparatus to determine not only Whether a weak light ash is present but,when it is present, also to determine its intensity and, furthermore,todetermine how its intensity varies with time. Moreover, this inventionseeks to provide means by which it is possible to determine theintensity of repetitive light ashes even Where the intensity is so lowthat, for example, only one photo-electron is emitted for every ashes.

It is also an object of this invention to provide apparatus for thedetection of light which because of its lo-w intensity has beenheretofore undetectable.

It is a further object4 of this invention to provide an improvedphotcmultiplier tube circuit wherein thephotomultiplier tube is operatedat voltages many times the ratedvdirect voltage without producinggas-generated noise therein. v

It is another object of this vinvention to provide high gain, e.g., ofthe order of 1000 times higher than with normal direct voltageoperation, in a-photomul'tiplie'r tube for a short duration of time. K

It is still another object of this invention to observe the lightintensity change during any particular time interval of a weak ash oflight.

Yet another object of the invention is to pulse-code communicationsystem.

A further object of the invention is to provide a lig'ht` pulse-objectdetection system.

It is still another object of the invention to provide a light pulsefriend or foe detection system.

Briey stated, in a circuit according to the invention, the electrodes ofthe photomultiplier tube are normally cut-off from all operatingvoltage, thereby rendering the tube normally inoperative, and onlyduring a short interval of time corresponding to the interval in whichthe incoming light intensity is to be observed, are voltages, which maybe several times greater than the rated direct voltage of the tube,impressed on the electrodes. By restricting the interval of time duringwhich the photomultiplier tube is on, to one of sutliciently shortduration, e.g., one micro-second or less, gas-generated noise current isunable to develop. This effect is obtained even at operating voltagesmany times greater than the rated direct voltage such that gain isappreciably stepped up while noise is maintained at substantially zerolevel. The mass of the positive ions is much greater than that of theelectrons. Therefore, during the interval in which the voltage is on thetube, these positive ions do not complete one transit between thedynodes and, consequently, cannot generate secondary electrons. l

The output of the photomultiplier tube is applied to an integratingcircuit and then it may be indicated, fr example, by means of anelectronic micro-ammeter. Consequently, with this pulsing techniqueconsiderably higher gains for a short duration of time are obtained inphotomultiplier tubes and very few thermalelectrons are received in theintegrating circuit; The thermal noise in the tube is reduced by afactor equal to the ratio of the time the voltage is on to the time itis otf.

In accordance with another aspect of this invention, the intensity of alight pulse, or any part thereof, as a function of time, is determinedby observing the output of a photornultiplier tube pulsed in accordancewith this inprovide a light vention as the tube pulsing operation isdisplaced in phase throughout the interval of the light pulse, or thedesired part thereof. l

Still further objects and advantages of the invention will be apparentfrom the following description and appended claims considered ingVdrawing, in which:

FIG. 1 is a schematic diagram of a Simplified embodi-r mentV of a pulsedphotornultiplier tube circuit in accordance with the invention;

i rate is not critical andv may be, for example, at'a rate Vwithin therange from about l to V100,000 pulsesV per,

together with the accompany- FIG, 2 illustrates one form of a pulsegenerator which may be used in theY circuit in accordance with theinvention; Y

FIG. 3 is a'more detailed diagram of another embodiment of a pulsedphotomultiplier tube circuitin accordance with the invention; v

FIG. 4 illustrates a light-beam communication system in accordance withthis invention;

FIG. 5 illustrates a light-beam ranging system in ac- Y lcordance withthis invention; and Y Y n FIG. 6 illustrates a light-beam identificationand/or signaling system.

VReferring to the drawings and more particularly to FIG.Y

1, in an arrangement according to the invention there'is i provided alight source 10, for example, a light emitting hydrogen thyratron' whichmay be controlled toproduce light pulses at a predetermined rate.

Means to activate'-V the light source, such as 'f1-variable trigger rategenerator 11 (e.g., an astable multivibrator), is coupled tothe lightsource 10 through conductor 12 to control the recurrence rate of thelight pulses. In this manner, which is known in the art, a light ilashhaving a duration of less than 1 microsecond may be produced. Y

Y The light-sensitive cathode 13a of a photomultipler tube 13 isVdisposed to receive'light from the light source 10. The photomultipliertube 13, illustrated in FIG. l, has nine dynodes, each designated byreference character 13b, disposed between the cathode 13a and an anode13e and it will be readily apparent that other types of photomultipliertubes may be substituted.V A voltage divider 14 having a plurality ofsimilar resistors 15, each having a low value of the order of, forexample, 100 ohms, is connected betweenthe'cathode 13a and a point ofground potential with intermediate connections or taps thereof connectedto the dynodes 13b.

A pulse generator 16, which produces photomultiplier tube operatingpulses, is connected to the cathode 13a Vof the photomultiplier tube 13,through a coupling capacitor 17 and,'furthermore, to the dynodes 13bthrough equally spaced taps on the voltage divider 14. 'I'he pulsegenerator 16 is preferably provided with means for controlling the pulseamplitude and theV pulse width.

The output voltage from the trigger rate generator 11, in addition tobeing applied to the light source l10, is also applied'as alsynchronizing impulse to the pulse generator variable delay device 18which provides meter or a recording potentiometer whenever-,Ja permanentrecord of the output signal is desired.

' 3,008,053 Y j i second.

During the time when the light ash is receivedrat the cathode ofphotomultiplier tube 13, a voltage pulse hav-Y Y ing an amplitude whichmay be manytimes the normaldirect voltage operating value'of thephotomultiplier tube .13 is applied to the electrodes of tube 13 todirect the flow of electrons from the photocathode 13a to Vthe anode 13evia the dynodes 13b. The voltage pulse-which is derived Vfromthe pulseYgenerator 16 may have a short duration of the orderV of one-hundredthof a microsecond, but the Vduration may not be shorter than the'transitL from the cathode to the anode of time fofV` the electrons the tube.

The. variable delay'device 18 may be adjusted so thatk the tube 13 isconducting only during a short interval of time corresponding to theinterval :in which the incoming light intensity is -to be'observed.` ThetubeV conducting period should'not exceed the time it takes to developgas-generated noise in the tube 13,A which varies with the voltageapplied Vtothe tube electrodes but which is usually Vabout Ionemicrosecond. The routput voltage from the photomultiplier tube 13 Vwhichis applied to the indicating device 21 through the integrating circuit22, Vis

substantially free fromv gas-generated and thermal noise. Although a'lightsourcel controlled by thenvariable trigger rate generatorllisillust'ratedinfthe drawing,

itis 4understoodthat the photomultiplier'tube circuitin accordance withthis invention may be used to detecty the intensity Yof any light whicheither-originates within-or withoutV the circuit, whether itbefconstant, variable or pulsatory light, by proper adjustments in theVvariable trigger rate generator 11 and the variable delay device 18 toeffect coincidence between the Vactivation of the photomultiplier tube13 andthe desiredY aspect of the incident light. f

` generator which may be used to-supplyivoltage pulses to thephotomultiplier tube 13 shoWn'in'FIG. Vl. This pulse,

generator comprises a conventional power supp-ly 23 which is capable ofproviding direct voltages of, say, from 0 to5,000 volts, a predeterminedlength of coaxial-cable 24,

" a-resistor 25 connected between the cable 24 and ground,

In the operation of the circuit illlustrated1nFIG. L y

it will be' seen that a light Hash from light source 10 is emitted,under the control of the trigger rate generator 11, and intercepted bythe light-sensitive cathode ofjthe photomultiplier tube 13. vThe rate ofthe voltage pulses from the pulse generator 16 is preferably equal tothe y' ate of ,the light pulses .fromtherlight source 10. This only abrief description.

a charging reactor 26 connected between the cable 24 and the powersupply 23 and a gaseous triode tube 27 connected to ground and the cable24 during conduction thereof.V The voltage at the control grid ofthegaseous Y triode tube 27 controls the repetition rate of the outputvoltage pulses developed across resistor 25. The duration of one ofthese'pulses is determinedY by the length of the coaxial cable `24'whichis chargedV by the voltageV from the power supply 23 With'the aid ofcharging reactor 26.

The voltage from the power supply 23 is applied to cable 24 so that eachtime when the gaseoustriodeltube 27 discharges, the shield of thecoaxial cable 24 becomesnegative and a negative pulse is developedVacross the resistor 25. lThis negative pulse can then be transmitted'tothe electrodes of the photomultiplier tube 13`through coupling capacitor17Vtok the voltage divider 14 show n in FIG. 1. YReferring now to FIG.3, whichis a more detailed'dia- Vgram of another embodiment of a pulsedphotomultiplier tube circuit in accordance with this invention, avariable frequency astable :multivibrator 30 having a twin-triode tube31 supplies `a triggering voltage to the light source` the operacircuit2S and to the light detector circuit 29, tion of which is similar to theabove-described operation ofthe circuit illustrated in FIG., l

Considering iist the light source circuit 28, it is seen that thetriggering voltage from the multivibrator 3,0 is applied to the controlgrid of the amplifier tube section 32.

The amplilied triggering voltage is then applied to rthe Y control gridof a cathode-follower ampliiier tube'section 33 in order toV impedancematch the integrating circuit FIG. 2 illustrates diagrammaticrally oneform of a pulse and therefore needs These principles have beenpreviously used in light-beam ranging devices. However, an objectionablefeature of type of system is that the intensities of reflected light areusually very low and, hence, heretofore have been undetectable.

In a light-beam ranging system, a fast rising light pulse must betransmitted to the reflecting object to obtain satisfactory resolution.This light pulse may be derived satisfactorily from a known light sourcecomprising a pulsed tube, for example, a thyratron, capable of producinga light pulse with a rise time of less than 0.1 microsecond, which maybe even as low as 0.01 microsecond. If 0.01 microsecond pulses are usedin a ranging device, the resolution is about plus or minus 1.5 meters.

In accordance with this invention, the reflected light is intercepted bya phototube which is pulsed on for approximately a 0.1 microsecondinterval of time at a predetermined time after the light pulse has beentransmitted to the reeeting object. The output of the pulsed phototubeis fed to a sensitive current indicating device through an integratingcircuit. The voltage pulses which pulse on the phototube are phased toproduce the maximum current at the output of the phototube, i.e., avoltage pulse is applied to the phototube at a time intervalcorresponding to the time in which a reflected light pulse is receivedat the phototube. The time interval between the instant at which thelight pulse is transmitted from the light source and the reflected pulseis received in the phototube, is a measure of the distance of thereflecting object, i.e.,

w-here d is the distance, c is the velocity of light, and l is the timeinterval.

The light pulse and the phototube may be pulsed repetitively at rates upto at least 20,000 pulses per second. By using this pulse technique andcurrent integration, it has been found that ten photoelectrons persecond can be readily detected. This means that only one effectivereected photon out of every 2,000 pulses is required to detect the delaytime of the reflected light-beam.

FIG. illustrates a light-beam ranging system in accordance withinvention. The trigger rate generator 11 connected to the light pulsesource 78 controls the transmitted light pulse repetition rate. Thetransmitted pulses 79 are intercepted by the photocathode 13a ofphotomultiplier tube 13 after they lare reflected by an object 80.

The trigger rate generator 11 is also connected to the photomultipliertube 13 through a variable delay device 18 and a serially connectedadjustable pulse generator 16 in the manner described hereinbefore withreference to FIG. 1. The variable delay device 18 is adjusted to aposition at which the reflected light pulses and the voltage pulses fromthe pulse 'generator 16 coincide in time at the photocathode 13a. Theoutput of the photomultiplier tube 13 is then fed from the anode 13e toan integrating circuit 83, to which is connected an indicating device84. The variable delay device 1S may be calibrated to give directreadings of distance between the photomultiplier tube 13 and thereflecting object 80.

In light-beam radar systems utilizing photomultiplier tubes operating ondirect voltage, light from the light pulse source which is scattered bythe atmosphere and objects in the immediate vicinity of the transmitter,produce a strong undesirable signal in the photomultiplier tube whichtends to overload the tube for an interval of time immediately followingthe transmission of each light pulse. The recovery time of thephotomultiplier tube after being overloaded is greater than the durationof the back-reflected light. A reflected light pulse received at thetube from the reflecting object during the recovery time is undetectabledue to the large and noisy output of the photornultiplier tube caused bythe back-scattered light. Since in the present invention thephotomultiplier tube is not pulsed on until the light pulse from the reflecting object is received, back-scattered light cannot overload thetube. Therefore, the present invention eliminates the :need for anopaque partition between the light pulse source and the receiver, whichis ordinarily necessary to minimize the effect of back-reflected lightwhen operating a photomultiplier tube in `a light-beam radar system ondirect voltage.

Yet another embodiment of this invention relates to a receiver in alight-beam identification and/or signaling system.

In a military operation i-t is very often desirable to identify anobject as friend or enemy. This can be accomplished by equipping allfriendly objects with a light source that can be pulsed on in any presetseries of pulses whenever a proper signal is received. This signal maybe received from a radio or light-wave receiver that is tuned to aproper frequency for receiving a signal from a central interrogatingtransmitter. Y

At the interrogating transmitter, or in line of sight o the interrogatedobject, a light receiver is disposed to receive the light pulses fromany object that is friendly. Enemy objects, not having the proper systemequipment, do not transmit the preset signal when interrogated and arethereby identified.

In accordance with this invention the light receiver includes aphotornultiplier tube having operating voltage pulses applied theretoonly during the time that a light pulse is anticipated. Consequently,the sequence of the operating voltage pulses corresponds with the presetcoded sequence of light pulses from the friendly light transmitter.

This system has the following advantages over a system using a detectorwhich is continuously sensitive: (1) great gain is obtained by pulseoperation; (2) a predetermined code can be used, thereby providinggreater security against enemy duplication; (3) the possibility of enemyjamming is almost completely eliminated; and (4) interference from otherlight flash sources, such as exploding shells, searchlights, etc., iseliminated. Furthermore, greater secrecy may be obtained by transmittinginfra-red or ultra-violet light instead of ordinary artificial ornatural light.

In FIG. 6 there is illustrated `a light beam identification system inaccordance with this invention. A synchronizing device 85, for example,a radio transmitter, either broadcast or directional, located preferablyat station #1, which may be -a central interrogating station, transmitsa signal which is received in receiver 86 of station #2. at theinterrogated object. A pulse generator 87, connected to the output ofreceiver 86, produces voltages pulses when a signal is lreceived by thereceiver 86. The voltage pulses from the generator i87 are passedthrough a coder 88 and applied to the light pulse source 89 in a codedsequence. A preset coded sequence of light pulses is then transmittedfrom the light pulse source 89 to station #l wherein the light pulsesare intercepted by the photocathode 13a of the photomultiplier tube 13.

The signal from the synchronizing device is also picked up by thereceiver 90 at station #l and applied to the pulse generator 91 whichsends voltage pulses through a variable delay device 92 to coder 93.This coder 93 applies a preset coded sequence of operating voltagepulses, corresponding to the sequence of light pulses, to thephotocathode-13a and dynodes 13b of the photomultiplier tube 13, whichis constructed and operated as described hereinbefore with reference toFIG. 1. The variable delay device 92 is Vadjusted to a position at whichthe light pulse sequence and the voltage pulse sequence are in phase atthe photocathode 13a of the photomultiplier tube 13. It can be readilyseen that the variable delay device 92 may be inserted in other parts ofthe system to produce the desired phasing between the light pulses andthe voltage pulses. The identification of friend or foe may be made, forexample, by means of au indicating device 94 34, 35 which is connectedacross resistor 36; The rheo- Y stat 34, which is part of theintegrating circuit 34, 3.5,. is used as a continuous variable delaydevice for the positive voltage which is derived from the resistor 36.When the positive voltage across capacitor 35 of the integrating cir-Vcuit 34, 35 reaches a predetermined'value, the gaseous Y. triodc orthyratron 37 ofthe pulse shaper r55is caused to fire. During the firingor conduction of the thyratron 37, a positive signal is taken fromacross'resistor 38 toptovide a positive'pulse to operate the blockingoscillator 39, which includes the Y triode section 40. The outp-ut ofthe blocking oscillator39 is taken yfrom the tertiary` winding 41 of theoscillator 39 and applied to thev control grid of anothercathode-follower amplifier tube section 42-which provides a lowimpedance trigger to the light-emitting hydrogen thyratron 43. Thefilter-'circuit 44' is inserted be: tween the thyratron 43 and Ytheblocking oscillator 39't'o prevent transients from vreilecting back tothe blocking oscillator 39 when'the thyratron 43 tires. Thepulse-'forming vnetwork 45 whichis coupledjtothe Vthyratron 43, controlstheV duration of the .light pulses emitted Y:from thyranature of themodulating signal are transmitted. An example of a known deltamodulation system wherein intelligence is conveyed via electricalYpulses is fully described in the Phi-lips Research Reports, volume .7,"pp. 442- 466,-December 1952, andfPhilips Technical Review,volume13,'pp.237-268March11952. y

Iii-accordance with, ,theV present invention, the delta modulated lightpulses `are radiated and intercepted'to establish a communication Inthis light-beam sysiteni the` receiverl is la light-,sensitive device,namely a Y photonuiltiplier tube, .which` produces an electrical pulseeach time a transmitted light pulse is received; V Thedetectoris madesensitiveto lightonly during the interval of time during whicha;ligl:tt'pulse expected. Ait other times, tllatY is',in' theinter'pulsetime,the light-sensu tive device is made inoperative and, therefore'.noise can'- n not enter the system during this time. 1

tron 43. The ariode'voltage for the thyratron 43 is fed Y through thehold-off' diode. 46,1 which Vprevents inverse voltage from getting backinto the power supply circuit, not shown, which is connectedat point47.` The pulseforming network 45 and thehold-oft' diode 46and associatedcircuitry are conventionally used ywith thyratrons and, therefore,further explanation is deemedunnecessary.

Now, considering the light detectorcir'cuit29, itis seen that thetriggering voltage `from the multivibrator j is also applied to thecontrol grid ofthe 'ampliiier tube section 48, The amplified triggeringvoltage is then applied to the vcontrol grid of a,cathode-follower'amplifier tube section 49 which provides a lowimpedance trigger through Vthe step delay line S0 to the thyratron 51.'f l'he step delay `line` 5l) may have a plurality; of `step,pos`itions,eg., three step positions, as shown in FLG. 3. Position a may representno delay; position bA one microsecond delay; and position 'c twomicroseconds delay. Thelength of coaxial cable 'S2 determines ,theVwidth or the output pulserrfrom The receiver in communicationsystem,which is illustrated in FIG. 4, iskconstructed.andoperated-similarlyto the circuit described iii-FIG.lfand, therefore', only Y 'abrief description is deemed` necessary. Y

"FIG, 4shows a transmitter57 traiismittingflightpulses 58 to a receiver59 which is synchronized with the trans'- mitter by `a'signal from thesynchronizing device 60 which may be, tor example, Va radio transmitterlor a-` transmissionV Line. The block diagramof transmitter 57Vinclu`desa pulse generator v61l u'rhichfsuppliesjav series'of identicalequidist-ant pulses 62 to the junctionpoint 63T through a r pruls'emodulator 64 with either unchanged -oropposite polarity'.V VThe actionofV the pulse *modulatorV 64 is gov'- erned bythe polarity ofthedifference voltage obtained ,y from the difference meter-65 bycomparison of the modthyratron 51 in a manner which Vis well-known inthe art. The positive pulse from Athyratron-Sl is then amplified in theamplifying tube 53 where itis inverted and applied as a negative pulseof high amplitude to the'photomultiplier tube 13 through capacitor 54.The photomultip-lier tube 13 and its output circuit, which includes theintegratiug'circuit ZZ'and the electron microarnineter 56, may

be similar in design and operation to the photomultiplier tube' and itsoutput circuit illustrated in FiG. l,

Although pulsed photomultiplier tubes are known in the art, thisinvention provides an improved photornu'l'ti'plier tube' circuit `whichcan determine the intensity Iof light which has a lower intensity thanany intensity heretofore detected and, furthermore, to determine theintensity of this light asa function of time.VV

From the foregoing description it` can be seen that thepresent'invention also provides an improved photomultiplier tube circuitin which the photomultiplier tube is s-ubstantially free fromgas-generated and' thermal noise even Y when-,operated at voltageshavinga Vmagnitude many timesthatof its rateddirect voltage.

inasmuch as the invention makes it' possible to detect light pulses ofextremely small intensity values, it will be evident that' the inventionhas'particular utility in connection with communication systems makinguse oflig'ht pulses whose periodicity, amplitudeduration or phase'position' is modulated'as a function of an intelligence or where fisthe reduction factor, PWA is the voltage pulseY Vulating Vsignal y66 andthe lstep-like approximating signal 67ffonm'ed ironrtheseries of pulsesV68by integratiorrvin the integrating network 69. The suppressor 70suppressesV the negative pulses'and passes the signal .'71`to the lightK v pulsesoua'cez'TZ which: produces light pulses581 corr'e-VV YspondingL to the electrical signal 71..

The iight pulses ss are intercepted-by the pimp-catliode 13a of thephotomultiplier tube V1 3 in Vthereceiver 59. The trigger rate:generator 1v1,- thevariablev delay device 18 and -the adjustablepulse'vgene'rator 16 aretinred andadjusted to apply a pulse voltage tothe photocathode 13a and dynodes 13b through the voltage divider Y14each time a light pulse isexpected atthe photocathodea. When theV signalpulses at theV anode of the tube 13 are applied to theintegnatingcircuit 22, asignal 7'3 is pro:-

duced thereacrosswhich is a close approximation of the originalsignal66. The signal appearing across theintegratin-gecircuit ZZ is suppliedto a utilizing device 76, for Vexample,a loudspeaker, via a low-passfilter 74y and amplifier 75-suppressing` inter alia the pulse.recurrence Y frequency. Other suitable circuits,` such las those de#`scribed in the above-'mentioned publications, may be coupled-to theoutputgofith'e tube to reproduce the original signal. Y y Y advantages.of this Vlight-beam communicatiousystem are:Y (l) no gas-generatednoise; `(2) thermal noise is reduced by the factor i width, and Pr isthe voltage pulse repetition rate; and (3) v extremely high gain ispossible.V v Ain ,important advantage of this communication system overother types of light-beam systems is its ability to Y operate when thelight signal received is so weak that only one photoelectron isY emittedfor each pulse received. c

Other systems, eg., amplitude modulation and pulse fre-Y quencymodulation, levels.VV

A still Yfurther embodiment of this' inventicnjrelatesto a light-beamranging system` forideteimining the distance to areeoting object inaccordanceV with radar principles."

require; considerably higher light such as an electron microammetenwhichis connected to the output of the photomultiplier tube 13 through theintegrating circuit 95.

It is to be understood that the above-described embodiments areillustrative of the application ot the principles of this invention.Numerous other embodiments may be devised by thoseY skilled inthe art towhich this invention pertains without departing from the scope thereof.

Having thus described my invention, what I claim and desire to secure byLetters Patent is:

1. In -a light detector, the combination comprising a photonlultipliertube including a plurality of dynodes, said tube having thecharacteristic of producing gas-generated noise signals when voltageslhaving relatively high values are applied to said dynodes for more thana relatively short period of time, resistance means connecting saiddynodes to va point of reference potential to render said tube normallyinoperative, a source of electrical pulses having a period of timeshorter than the time in which said gas-generated noise is normallydeveloped in said tube, and means connected to apply said pulses to saiddynodes.

2. In a light detector, the combination comprising a photomultipliertube including a plurality of dynodes and a cathode, said tube havingpredetermined operating voltage ratings for said dynodes and saidcathode, said tube having the characteristic of producing gas-generatednoise signals when voltages having relatively higher Values than saidvoltage ratings are applied to said cathode and dynodes for more than arelatively short period of time, resistance means connecting saiddynodes and cathode to a point of reference potential to bias saidcathode and dynodes at a value to render said tube normally inoperative,means providing direct voltage operating pulses for said dynodes andsaid cathode having values substantially in excess of said voltageratings for a period of time shorter than the time in which saidgas-generated noise is developed in said tube, and means connected toapply said pulses to said dynodes and said cathode, respectively.

3. A light detector comprising a photomultiplier tube having electrodesincluding a photocathode, a plurality of dynodes and an anode, saidftube having the characteristic of producing gas-generated noise signalswhen voltages having relatively high values are applied to said dynodesfor more than a relatively short period of time, a. plurality ofseries-connected resistors having one end connected to saidphotocathode, the other end connected to ground and the junctionstherebetween connected respectively to said dynodes, an indicatingdevice, an integrating circuit coupling said device to said anode, and asource of unidirectional pulse voltages having a predetermined low valueand a predetermined high value coupled to the electrodes of saidphotomultiplier tube through said series-connected resistors fordirecting the ow of electrons from said photocathode via said dynodes tosaid anode, the duration of each of said unidirectional pulse voltagesbeing less than the time in which said gas-generated noise is developedin said photomultiplier tube, said low value being of a magnitudeproducing substantially cut-oi conditions in said tube.

4. A light detector, `as set forth in claim 3, includingvoltage-determining means connected to control said values of pulsevoltage whereby said predetermined low value is zero and saidpredetermined high value is subswantally greater than the rated directvoltage for the electrodes of said photomultiplier tube, and includingmeans connected to control the duration of said pulse voltages wherebythe duration of each of said pulse voltages is not greater than onemicrosecond.

5. A light detector comprising a photomultiplier tube having aphotocathode, a plurality of dynodes `and an anode, said tube having thecharacteristic of producing gas-generated noise signals when voltageshaving relatively high values are applied to said dynodes for more thana relatively short period of time, a voltage divider having a pluralityof equally spaced tapsconnected between said photocathode and ground,said taps being respectively connected to said dynodes, said voltagedividerv being adapted to bias said cathode and dynodes at a value torender said tube normally inoperative, a current-sensitive device, anintegrating circuit coupling said device to said anode, a pulsegenerator coupled to said photomultipl-ier tube through said voltagedivider for applying to said photocathode and said dynodes voltagepulses having a magnitude substantially greater than the magnitude ofthe rated direct voltage of said photomultiplier tube for directing theow of electrons from said photocathode to said anode, the duration ofeach of said pulse voltages being less than the time in which saidgas-generated noise is developed and being not greater than onemicrosecond, and a variable trigger rate generator coupled to said pulsegenerator to control the pulse repetition rate thereof.

6. A light detection system comprising a photomultiplier tube having aphotocathode, a plurality of dynodes and an anode, said tube having lthecharacteristic of producing gas-generated noise signals when voltageshaving relatively high values are applied to said dynodes for more thana relatively short period of time, a plurality of series-connectedresistors having one end connected to said photocathode, the other endconnected to ground 'and the junctions therebetween connectedrespectively to said dynodes, the series resistor connection betweensaid dynodes and cathode and ground providing a bias on said dynodes andcathode that renders said tube normally inoperative, an indicatingdevice, an integrating circuit coupling said device to` said anode, apulse ygenerator coupled to said series-connected resistors for applyingto said photocathode and said dynodes voltage pulses having a magnitudemany times the magnitude of the rated operative direct-current voltageof said photomultiplier tube, the duration of each of said pulsevoltages being less than the time in which said gas-generated noise isdeveloped and being not greater than one microsecond, a variable triggerrate generator coupled to said pulse generator to control the pulserepetition rate of the voltage pulses, a light source for producingpulsatory light and disposed to transmit said light to saidphotocathode, said variable trigger rate generator being coupled to saidsource to control the repetition rate of said pulsatory light, and avariable delay device interposed between said trigger rate and saidpulse generators for phasing the voltage pulses from said pulsegenerator with respect to the light pulses from said light source.

7. A light detection system comprising Ia variable trigger rategenerator producing voltage pulses, a first pulse amplifier coupled tothe output of said generator, a rst pulse Shaper, a continuous variabledelay device coupling said amplifier lto said shaper, =a blockingoscillator coupled to the output of said Shaper, alight sourcecomprising a gas discharge tube, a lter circuit coupling the output ofsaid blocking oscillator to said light source for preventing transientsfrom retlecting back to said blocking oscillator, a second pulseamplifier coupled to the output of said variable trigger rate generator,a second pulse shaper providing shaped pulses each of which has aduration less than one microsecond, a step delay device coupling saidsecond amplier to said second shaper, a photomultiplier tube having aphotocathode, an anode and a plurality of dynodes, said tube having thecharacteristic of producing gas-generated noise signals when voltageshaving relatively high values are applied to said dynodes for more thana relatively short period of time, said photo cathode being disposed toreceive light from said light source, a plurality of series-connectedresistors having one end connected to said photocathode, the other endconnected to ground and the junctions therebetween connectedrespectively to said dynodes, the series resistor connection betweensaid photocathode, dynodes and ground providing a bias on said dynodesthat renders Yamplifying and applying said shaped pulses Withga nega- Ysaid. tube nQrmally inoperative, a ,thi'd pulse amplifier,

tive polarity to said photocarthode and throughY said re-V sistor tosaid dynodestqrender said Vtube operativeY Vin a pulsatory manner, saidshaped pulses having durations less than the .time in `which saidgas-'generated rnoise Ais developed, a' current-sensitive device, andian integrating circuit coupling said current-sensitive device to saidanode.

References Cited in the lile of patent `UNITED STATES PATENTS 2,234,329`Wolff Mar. 11, 1941 v2,524,807 Y2,538,062 n ,1 2,594,703 Y Handen Nov.5, 1957

